example.js

function getSelectedLayersIdx() {
	var selectedLayers = new Array();
	var ref = new ActionReference();

	ref.putEnumerated(
		charIDToTypeID("Dcmn"), 
		charIDToTypeID("Ordn"), 
		charIDToTypeID("Trgt")
	);

	var desc = executeActionGet(ref);
	if (desc.hasKey(stringIDToTypeID('targetLayers'))) {
	
		desc = desc.getList(stringIDToTypeID('targetLayers'));
		for (var i = 0; i < desc.count; i++) {
			try {
				activeDocument.backgroundLayer;
				selectedLayers.push(desc.getReference(i).getIndex());
			} catch(e) {
				selectedLayers.push(desc.getReference(i).getIndex() + 1);
			}
		}
	
	} else {

		var ref = new ActionReference();
		ref.putProperty(
			charIDToTypeID("Prpr"), 
			charIDToTypeID("ItmI")
		);
		ref.putEnumerated(
			charIDToTypeID("Lyr "), 
			charIDToTypeID("Ordn"), 
			charIDToTypeID("Trgt")
		);
		
		try {
			activeDocument.backgroundLayer;
			selectedLayers.push(
				executeActionGet(ref).getInteger(charIDToTypeID("ItmI")) - 1
			);
		} catch(e) {
			selectedLayers.push(
				executeActionGet(ref).getInteger(charIDToTypeID("ItmI"))
			);
		}

		var vis = app.activeDocument.activeLayer.visible;
		if (vis == true) {
			app.activeDocument.activeLayer.visible = false;
		}

		var desc9 = new ActionDescriptor();
		var list9 = new ActionList();
		var ref9 = new ActionReference();
		
		ref9.putEnumerated(
			charIDToTypeID('Lyr '), 
			charIDToTypeID('Ordn'), 
			charIDToTypeID('Trgt')
		);
		list9.putReference(ref9);
		desc9.putList(charIDToTypeID('null'), list9);
		
		executeAction(
			charIDToTypeID('Shw '),
			desc9,
			DialogModes.NO
		);
		
		if (app.activeDocument.activeLayer.visible == false) { 
			selectedLayers.shift();
		}
		app.activeDocument.activeLayer.visible = vis;
	}
	return selectedLayers;
};